This invention relates to sewing machines and, more particularly, to an optical switching arrangement within a sewing machine.
There are a number of known optical switching arrangements for sewing machines in the prior art. For example, such arrangements are known for signalling a sewing machine operator as to the impending depletion of bobbin thread. This is desirable in order to warn an operator of impending bobbin thread exhaustion which might interfere with the appearance of a long seam. A number of these arrangements utilize a light source and a light detector arranged so that when there is thread on the bobbin the optical path from the light source to the light detector is blocked, this path being opened when the amount of thread remaining on the bobbin is depleted below some threshold value. Upon the occurrence of this latter condition, appropriate circuitry activates an alarm, or indicator, that warns the operator that the amount of thread remaining on the bobbin is below the predetermined threshold. Many of these arrangements have frequently been of limited reliability due to the effect of stray light producing erroneous triggering of the low bobbin thread warning alarm. Most attempts to eliminate the effect of stray light from such sources as room lighting and sewing machine mounted work guiding lights have been limited to enclosing the loop taker cavity in which the light sensitive photo detector resides with an opaque bed slide and spraying the cavity with a flat black paint treatment to reduce internal reflections from reaching the light detector. Other attempts have involved defining a narrow optical path from the light source to the light detector and providing shielding about the lift detector to block all light other than that which emanates from the light source. While these prior arrangements are somewhat effective, they still allow some amount of ambient light to penetrate and they have the further problem that the light level is diminished by the shielding. Further, most of these prior arrangements utilize a relatively expensive subminiature incandescent lamp.
Another application utilizing optical switching in a sewing machine is for buttonholing. In particular, a movable buttonhole foot may carry reflective means, the movement of which is sensed as the foot is moved by the fabric feeding movement of a feed dog against the garment being sewn. Mounted on the sewing head is a light source and light detector, both focused down toward the reflective means. It is apparent that the light detector is exposed to stray ambient light.
It is therefore an object of the present invention to provide an improved optical switching arrangement for a sewing machine.
It is a further object of this invention to provide a low bobbin thread detection and indication system which is both reliable and cost effective.
It is another object of this invention to provide an optical buttonhole switching arrangement for a sewing machine which is both reliable and cost effective.